Choosing Lines for Comparison
Rather than compare the entire available spectrum, we suspect that selecting specific lines could produce better results. Nick found an article by Fuhrmann et al. (1997) which suggests that the Mg Ib lines are good indicators of surface gravity in F and G stars. The spectra are full of Fe I and Fe II lines, but these are "susceptible to overionization effects and uncertainties in the temperature structure of [their] model atmosphere." Below are plots of the Mg triplet over a range of surface gravities and temperatures appropriate for our purposes. I also include a sample of 9020's triplet for comparison.
The Mg lines are often the strongest lines in our spectra, and their profiles vary smoothly over the paramter ranges of interest. We will have to think carefull about how to treat the triplet line with the shortest wavelength because it is very close to several Fe lines of comparable strength. As Bob noted, the above plots show that there is nothing intrinsically wrong with using an Fe line. We will have to wait to see if twinsyn has difficulty working at the level of detail necessary to handle that busy section of the spectra. It is important say that the above synthetic spectra were not produced from the grids Rich will use in twinsyn. Instead they were part of a catalogue provided by Soeren Meibom at the Harvard-Smithsonian Center for Astrophysics.